"Normally", if you're inloged with your username on the site, you'll be able to login in the chat without problem.

The chat is using IRC freenode.net, that's not my server, but "many" channels and groups are using it.
So there is a chance someone is using your nickname over there already.
When you login with the webchat here, SFS_ is added before your username.
If not available, you'll have to use an other username in the chat.